The present invention relates to a novel process for the recovery of iodine from solutions containing iodinated organic compounds, in particular non-ionic contrast agents.
The most modern contrast agents, i.e. the non-ionic ones, are usually 2,4,6-triiodo-1,3-benzenedicarboxylic acid derivatives, characterized by a strong bond of the iodine atoms to the aromatic ring. The strength of said bond is however also affected by the structure of the concerned compound.
Due to environmental reasons, the effluents should be completely free from iodinated organic compounds, while for economic reasons as much iodine as possible should be recovered, in that it is now rare and expensive. In this case, economic and environmental needs meet.
The problem has arisen for some time, as evidenced by a series of patents concerning this process (see for example: WO 98/07661; WO 94/10083; NO 9100001; EP 106934). In particular, EP 106934, filed in the Applicant"" name in 1982, discloses the process for the mineralisation of iodine, which comprises heating the solutions of the contrast agents for 30 minutes-2 hours at 100-150xc2x0 C. in a strong alkali excess in the presence of 100 to 2500 ppm of copper ions or of finely dispersed copper.
The recovery of iodine after mineralisation is carried out according to known methods, usually by oxidation of the formed iodide with mild means. The formed iodine is recovered and concentrated, for example, by extraction or by sublimation in air or vapour stream. Iodine can also be recovered from the extraction solvent (for example toluene) by treatment with aqueous alkali hydroxide, and the iodine vapours obtained in the sublimation can be collected, for example, through absorption in alkali hydroxide solutions by iodide-iodate dismutation.
Iodine is known to quantitatively regenerate from the alkali hydroxide solution by acidification following dismutation.
It can easily be evinced from EP 106934 (page 6, Table) that the temperature of the mineralisation process depends on the type of the contrast agent.
The conditions for the preparation of Iopamidol and Metrizamide, which then represented the novel class of non-ionic compounds (i.e. in which the carboxylic groups are in the form of amides with aminoalcohols), are more drastic than those for the ionic compounds, i.e. those having at least one acid group on the aromatic ring (acetrizoic, diatrizoic, iothalamic, iopronic, iopanoic acids, iodamide and adipiodone). For example, the best results are achieved when heating the 2% Iopamidol solution to 150xc2x0 C. in the presence of 2500 ppm of copper ion.
The most diffused compounds are at present the non-ionic ones. The preparation thereof usually involves crystallizations from a solvent, usually alcoholic, both in the intermediate steps and the final one.
The presence of solvents in wastes from which iodine has to be recovered makes carring out the cited process more difficult, particularly in the following steps:
when the mineralisation process is carried out, according to the teaching of the Patent, at least at 100xc2x0 C., it is difficult to reach said temperature due to the low-boiling organic solvent (such as a lower alcohol), unless operating under pressure;
the presence of the solvents, in particular alcohols, makes the precipitation of iodine after sublimation problematic, unless using a system of scrubber;
when using an alternative process for recovering the sublimated iodine (treatment with alkali and subsequent precipitation of iodine by acidification), side-formation of iodoform takes places due to the presence of compounds having RCOCH3/RCHOHCH3 groups. 
Said reaction, which has to be absolutely avoided for environmental reasons, also prevents the recovery of iodine due to the precipitation of iodoform.
finally, it is necessary to increase the amount of oxidizer necessary to oxidize the iodide formed after mineralisation.
A further technical problem which had not been evidenced in the above Patent is that, after the mineralisation step, phenol or quinone organic compounds are present which cause a consumption of oxidizer in the subsequent oxidation step and moreover have a high environmental impact, in that they are not biodegradable.
It has now surprisingly been found that the above cited problems can be solved by concentrating the solution obtained under the mineralisation conditions described in EP 106,934 and purifying said solution by nanofiltration before the oxidative step, thereby improving the overall yield of process.
It is therefore an object of the present invention a process for the recovery of iodine from mother liquors or wastes with a percentage of organic solvents at most of 95% (w/w), containing iodinated organic compounds, by mineralisation of organic iodine (in the presence of copper ions or finely dispersed metallic copper in alkali aqueous solution) and subsequent transformation of the formed iodide into elementary iodine, characterized in that the aqueous solution, after mineralisation, is concentrated under atmospheric pressure and at the boiling temperature and is subsequently subjected to nanofiltration.
The process is particularly suitable for the treatment of solutions of non-ionic iodinated contrast agents, such as: Iopamidol, Iohexol, Iopromide, Ioxilan, Iomeprol, Iopentol, Ioversol. The process of the invention can also be applied to ionic contrast agents, if the above stated conditions are fulfilled. In this case, aqueous or organic solvents solutions will be present, depending on compound or waste to be treated and on the synthetic step.
The solution is preferably adjusted to a volume ranging from 85% to 25% (w/w) of the starting volume. Said procedure is easily applicable to waste solutions from the synthesis of ionic or non-ionic contrast agents, possibly in the presence of solvents, and it allows to decrease the COD from values of 20.000-40.000 mg/L to 4.000-9.000 mg/L through demolition of the organic molecules present and removal of any solvents.
The conditions of the mineralisation process of the present invention are the same as those disclosed in EP 106,934.
The copper catalyst is added in amounts from 100 to 3000 ppm, preferably 500-1000 ppm, and pH is kept at 12 during the whole mineralisation step.
The process of the invention comprises heating the solution to be deiodinated at the boiling temperature and under atmospheric pressure.
This, of course, involves rather long times such as:
4 to 6 hours for mineralising a solution of Iopamidol, Iomeprol, Iohexol, Metrizamide or of a generic non-ionic contrast agent, in concentration of 2%-10% (w/w) with an at most 95% (w/w) content in alcoholic solvent;
2 to 3 hours for mineralising a solution of the mother liquors from the production of Iopamidol intermediates containing 3%-15% (w/w) of iodinated intermediate, with the following maximum contents (w/w): 2-butanol 20%, methylchloroform 5%, n-butyl acetate 4%, n-dodecane 3% (w/w) and tert-butanol 30%. As mentioned above, the process of the invention can also be used for mother liquors from production cycles of other ionic contrast agents such as:
3,5-acetylamino-2,4,6-triiodo-benzoic acid sodium salt (DIAC), acetrizoic acid (3-acetamino-2,4,6-triiodobenzoic acid);
adipione (3,3xe2x80x2-[(I,1,6-dioxo-1,6-hexanediyl)diimino]-bis-2,4,6-triiodobenzoic acid;
iodossamic acid (3,3xe2x80x2-[(1,16-dioxo-4,7,10,13-tetraoxahexadecane-1,16-diyl)diimino]-bis-2,4,6-triiodo benzoic acid;
iothalamic acid (3-(acetylamino)-2,4,6-triiodo-5-[(methylamino)carbonyl]-benzoic acid;
iopronic acid (2-[[2-[3-(acetylamino-2,4,6-triiodo-phenoxy]ethoxy] methyl]butanoic acid;
iopanoic acid, 3-amino-xcex1-ethyl-2,4,6-triiodobenzenepropionic acid.
The above Patent did not envisage any concentration or elimination step of the organic solvents (see examples 1, 2, 4, 5, 7, 27, 55).
In Examples 3 and 6, concerning non-ionic contrast agents (Iopamidol and Metrizamide) it is even necessary to use an autoclave or a sealed container to carry out the mineralisation at 130-150xc2x0 C. for 1 hour.
Conversely, a feature of the process of the invention is a concentration step under atmospheric pressure, which avoids the use of an autoclave, making use of the ebullioscopic raising due to the solids present at 105-120xc2x0 C. and increasing the duration of the step by 2-6 hours. When the solutions do not contain salts dissolved so as to guarantee the ebullioscopic raising, sodium sulfate and/or sodium chloride can be added to produce such an effect. The amounts to be added depend on the mixture to be mineralised.
Operating this way, any solvents present are removed by either direct or vapour stream distillation while carrying out the mineralisation of iodine.
This is particularly important in the case of lower alcohols which are the most used solvents in the processes for the preparation of non-ionic contrast agents which comprise a final crystallization and which can give rise to iodoform, as already mentioned.
Furthermore, the solution to be fed to the subsequent step is concentrated, which is particularly useful in case it allows to remove some inorganic salts less soluble than sodium iodide.
This simple operation provides a significant decrease in the contamination load with an increase in biodegradability. For example, in a production waste of Iopamidol, starting from a COD of 25.000 mg/L and a BOD of 1.500 mg/L, a solution can be obtained with COD 7.000 and BOD 3.500 at the end of the mineralisation. This reduction is due the elimination of the solvents and to the partial degradation of the aromatic molecules.
It is also possible to include a filtration step of the solution after mineralisation and a concentration step when high concentrations of poorly soluble saline compounds are present, which can precipitate in the concentration step (in particular the precipitation of Na2SO4xc2x710 H2O can take place).
Filtration provides the advantage of removing part of the ionic compounds present in solution, thereby reducing the osmotic pressure in the subsequent nanofiltration step.
The solution, after mineralisation, concentration and optional filtration, is purified by nanofiltration. Said operation is carried out in two steps:
1. nanofiltration of the solution;
2. diafiltration with addition of water so as to adjust the permeated solution to about the starting volume before the mineralisation step.
The process of the present invention surprisingly employs this technique keeping the permeate instead of the retentate, which is more commonly the purified fraction. In this case, the solution from which the iodine is recovered is the permeate, mainly containing sodium iodide, purified from the high molecular organic substances and from sodium sulfate and any sodium sulfite.
The iodide concentration of the permeate ranges from 0.6 to 1.4%, evaluated by argentometric titration.
The final step for the recovery of solid iodine is effected conventionally, as cited above.
Particularly preferred is the use of hydrogen peroxide as oxidizer, at pH 0.5-1.5 by addition of 50% w/w sulfuric acid, at room or high temperature (20-50xc2x0 C.), mainly in that the reduced product is water.
The almost complete absence of organic molecules advantageously reduces the amount of oxidizer necessary. In fact, in the process of the invention, only a slight excess (5-15%) to the stoichiometric is envisaged.
The oxidation is substantially instantaneous, as no oxidizable side-products are present, and is monitored by measuring the ox-redox potential with a platinum electrode.
The reaction is considered completed at a 480-540 mV of calomel-relative potential.
The last step is the recovery of iodine by two ways:
1. Filtration of the precipitated iodine, which is possible, contrary to the prior art, in that only NaCl is present in the solution. To improve precipitation yields the solution may be concentrated to 30-50% of the starting volume before the oxidation, guarantying an about 3% iodine content. Total yields are about 88-90% on theoretical.
2. Sublimation of precipitated iodine in vapour stream, absorption in alkali (preferably 30% NaOH) as Ixe2x88x92/IO3xe2x88x92 at a iodine maximum concentration of 3.5% and precipitation by acidification. In this case, total yields are about 90-95% on theoretical.
According to a further aspect of the invention the oxidation and recovery of iodine are carried out in a single step.
The permeate from the nanofiltration step is acidified to pH 0.5-1 with 50% sulfuric acid, at temperatures ranging from 20xc2x0 C. to 50xc2x0 C. Said solution is fed to a continuous extractor adding the oxidizer in line so as to guarantee a permanence time sufficient for the complete oxidation and extraction. Iodine is extracted with a suitable solvent (such as toluene, methyl tert-butyl ether, dodecane). This way iodide is oxidized to iodine and extracted from the aqueous solution in a single industrial operation. Iodine is extracted from the solvent by treatment with bases and then recovered as already described above. The overall yield is, in this case, about 93-96% on theoretical.